1. Field of the Invention
This invention relates to catalyst reactivation methods. More particularly, this invention relates to methods for reactivating totally or partially deactivated alkylation catalysts with a reactive near-critical, critical or supercritical fluid reactivating agent.
2. Related Technology
Nomenclature
As conventionally accepted in the literature on alkylation, terms such as alkanes, paraffins and paraffinic hydrocarbons will hereinafter refer to open-chain saturated hydrocarbons. The suffix -ene is adopted for straight-chain monounsaturated hydrocarbons, so that a term such as butene refers to at least one of the compounds 1-butene and 2-butene. The suffix -ylene is hereinafter employed to refer to a monounsaturated hydrocarbon that consists of the same number of carbon atoms as expressed by the name. For example, the term butylene refers to at least one of the compounds 1-butene, 2-butene, and isobutylene, the latter compound also is known as 2-methylpropene. Terms such as alkenes, olefins and olefinic hydrocarbons generically refer to monounsaturated hydrocarbons.
The prefix iso- is generically used to refer to branched alkanes or alkenes that have one or more methyl groups only as side chains. Aromatic hydrocarbons refer to hydrocarbons that have at least one aromatic ring and to hydrocarbons which, although strictly not aromatic, contain conjugation to the extent such that they undergo alkylation reactions like aromatic compounds.
The term Cn describes a hydrocarbon with n carbon atoms, whether the hydrocarbon is linear, branched, paraffinic, olefinic or aromatic. The notation Cn-Cm describes at least one hydrocarbon in the set of hydrocarbons such that the number of carbon atoms ranges from n to m for any individual hydrocarbon in the set. The notation Cn≧p or Cp+ refers to at least one hydrocarbon with at least p carbon atoms, and it often refers to a mixture of hydrocarbons such that the number of carbon atoms is at least p′ for any individual hydrocarbon in the mixture.
Processes
The term alkylation generically refers to the addition of an alkyl group to a molecule that is to be alkylated. Alkylation of alkenes to produce alkylation products, or alkylate, is an addition of a saturated hydrocarbon (R—H) to an alkene to yield a saturated hydrocarbon of higher molar mass. This reaction is generically represented by the following chemical equation: 
Alkylation is extensively used in the petroleum industry to produce medium- or large-mass hydrocarbons from smaller molecules. One of the more important alkylation reactions is the addition of isobutane to 2-butene to produce 2,2,4-trimethylpentane according to the following equation: This reaction is conventionally carried out in the presence of an acid such as sulfuric acid or anhydrous hydrofluoric acid.
According to the nomenclature previously introduced, the first reactant in equation (1) is an alkane, paraffin or paraffinic hydrocarbon, whereas the second reactant in the same equation is an alkene, olefin, or olefinic hydrocarbon that can also correspond in that equation with an alkylene. More specifically, the paraffin which is listed as the first reactant in equation (2) is isobutane, and the alkylene which is listed as the second reactant in the same equation is 2-butene. Furthermore, equations (1) and (2) describe with varying degrees of generality paraffin alkylation, or the addition reaction of a paraffin and an olefin. Equation (2), in particular, describes the addition reaction of an isoparaffin and an olefin where the alkylate is an isoalkane.
The notation used in equation (1) describes a reaction that includes, for example, the reaction of a C4-C8 paraffinic hydrocarbon with a C2-C12 olefinic hydrocarbon to produce a branched paraffinic hydrocarbon. In the particular example provided by equation (2), a C4 isoparaffin reacts with a C4 olefin to produce a C8 isoparaffin.
As indicated above, aromatic hydrocarbons can also be alkylated. For example, benzene can be alkylated with ethylene to produce ethylbenzene, a precursor of styrene, according to the zeolite catalyzed reaction that is described by equation (3): Ethylbenzene yields, upon dehydrogenation, styrene, which is the simplest and most important member of a series of unsaturated aromatic compounds. The zeolite-catalyzed alkylation of benzene by ethylene has been described in a number of sources. See, for example, Kirk-Othmer Encyclopedia of Chemical Technology, Vol.21, pp. 770-800, 3rd ed. (1983).
The olefins in equations (1)-(3) are the respective alkylating agents. Generally, in alkylation reactions, the amount of the reactant to be alkylated exceeds the amount of the alkylating agent. Thus, when an aromatic hydrocarbon is alkylated with an olefin, it is preferred to operate with a molar ratio of the aromatic hydrocarbon to the olefin greater than 1:1, and preferably from about 2:1 to 5:1 as measured by the flow rates into the reaction zone. Similarly, it is preferable to operate with a paraffin-to-olefin molar ratio greater than 2:1. Preferably, the paraffin-to-olefin molar ratio exceeds 3:1. However, ratios as high as 100:1 can be employed. The use of a large-pore zeolite with a Lewis acid reportedly increases the activity and selectivity of the zeolite, thus permitting effective alkylation at high olefin weight hour space velocity (OWHSV) and low isoparaffin/olefin ratio. The OWHSV is defined as the amount of olefin fed to the reactor per unit catalyst per hour (i.e., g olefin (g catalyst)−1 h−1).
The principal industrial application of paraffin alkylation is in the production of premium-quality fuels for internal combustion engines. More specifically, alkylation is mainly used to provide a high octane blending alkylate for automotive fuels that also increases the fuel sensitivity to octane-enhancing agents. Alkylate components are typically characterized by clean, low emission burning. Because of these properties, alkylate production capacity is expected to increase as specifications for gasoline become more stringent.
Most commercial alkylations rely on catalytic processes for the production of alkylate. Catalysts used in industrial alkylations have typically been strong liquid acids, such as sulfuric acid and hydrofluoric acid. Other strong acids have been used in laboratory or industrial alkylations. These acids include aluminum trichloride, and super acids such as trifluoromethanesulfonic acid.
In addition to problems related to undesired polymerization and side-reactions, liquid acid alkylation requires the use of a fairly concentrated acid and the replacement of consumed acid. For example, sulfuric acid concentration is controlled above 90% to provide optimum activity and selectivity, and hydrofluoric acid concentration is maintained in the range of 85-95%. These acids, however, are recognized hazardous materials, whose use requires the adoption of periodic hazard reviews of the operating units and the implementation of safety procedures and measures to minimize the probability of accidental releases. Other typically costly measures that must be adopted include control operations to mitigate the detrimental effects of such possible accidents.
Another drawback of the use of liquid acid catalysts is the disposal of sludge formed during alkylation. This waste sludge that is produced by sulfuric acid or hydrofluoric acid catalyzed alkylations is subject to stringent environmental regulations. The regulated waste management operations for the disposal of this sludge add considerable expenses to commercial alkylation.
The residue known as “red oil” is another product derived form liquid acid catalysis that presents disposal and recycling problems. Red oil is predominantly the conjugation product of an acid and alkylate that has to be disposed of, or recycled. Disposal presents a problem that is inherent in the storage, handling and deposit of hazardous substances. Further, recycling is also an expensive operation because it requires the implementation of additional processes that significantly increase the cost of producing the desired alkylate.
The handling and disposal problems associated with liquid acid alkylation catalysts cause technological developments in alkylations to be greatly influenced by environmental considerations. One reason is that modern low emission gasoline formulations rely heavily on alkylate. Furthermore, as noted in the foregoing discussion, the use of liquid acid alkylation catalysts requires a constant improvement of process safety, the reduction of waste disposal, and the limitation of the environmental consequences of any process emissions. In addition, liquid acid catalysis employing sulfuric acid or hydrofluoric acid is not an effective means for catalyzing certain alkylations, such as the alkylation of benzene with ethylene.
It is therefore desirable to provide alkylation catalysts which can be used in the production of low emission fuels, which are noncorrosive and easy to handle, and which can be effectively reactivated to avoid disposal problems. Because solid acid catalysts are easier to handle and less hazardous than liquid acid catalysts, they are good candidates to replace liquid acid alkylation catalysts. However, solid catalysts are known to deactivate relatively rapidly as a consequence of fouling of the active sites by heavy reaction intermediate products and byproducts. This is considered a major hurdle for the effective use of solid acid alkylation catalysis. See Kirk-Othmer, Encyclopedia of Chemical Technology, Vol.2, p.92, 4th ed. (1991). Rapid deactivation of solid acid catalysts leads to relatively large volumes of material that must be discarded. Disposal of such material introduces a host of complications, such as environmental issues and the like. See id., p. 108. Consequently, it is particularly desirable to provide solid acid alkylation catalysts which can be handled easily, and which can easily be reactivated to an active condition so that they can be used effectively in further alkylation reactions.
Because alkylation reactions typically take place in a fluid medium, the use of solid acid catalysts is also referred to as heterogeneous catalysis.
Heterogeneous Catalysis
The term “catalyst” as used herein includes any solid catalyst with sufficient strength to carry out alkylations. A large number of alkylation catalysts have been proposed, including molecular sieves, and in particular zeolites, silicates, aluminophosphates, and silicoaluminophosphates. Alkylation catalysts can be chosen from among a variety of substances, with the specific catalyst often determined by the character of the processes carried out in the plant where the alkylation takes place.
Zeolites, which can be natural, synthetic or mixtures thereof used as catalysts in alkylations include ZSM-4, ZSM-3, ZSM-5, ZSM-20, ZSM-18, ZSM-12, ZSM-35, ZSM-48, ZSM-50, MCM-22, PSH-3, TMA offretite, TEA mordenite, REY, faujasites comprising zeolite Y and mordenite, ultrastable Y zeolites (USY), and a number of zeolites such as zeolite beta, zeolite Omega, zeolite L, and clinoptilolite, and rare-earth metal containing forms of zeolites. Other catalysts include at least one among a variety of inorganic oxides such as alumina, and in particular η or γ alumina, silica, boria, phosphorous oxides, titanium dioxide, zirconium dioxide, chromia, zinc oxide, magnesia, calcium oxide, silica-alumina, silica-magnesia, silica-alumina-magnesia, silica-alumina-zirconia, sulfated mixed-metal oxides, and more generally a variety of refractory inorganic oxides and natural substances such as bauxite, clay, including kaolin and bentonite, and diatomaceous earth. Molecular sieves that also catalyze alkylations include pillared silicates and/or clays, aluminophosphates such as ALPO-5 and VPI-5; silicoaluminophosphates such as SAPO-5, SAPO-37, SAPO-3 1, SAPO-40, and SAPO-41,other metal aluminophosphates, and layered materials such as MCM-36. These catalysts, alone or in combination among themselves or with other substances are known to be used in alkylations of olefins and aromatic hydrocarbons. For example, one of the non-zeolitic substances that can be combined with zeolites in the preparation of alkylation catalysts is at least one Lewis acid, such as boron trifluoride, antimony pentafluoride, and aluminum trichloride. Refractory oxides can be used in combination with other catalytic substances to provide temperature resistance. In addition, diluent materials such as various oxides and clays can be incorporated to control the conversion rate, to improve the catalyst's mechanical properties, to provide a matrix material, and/or to act as catalyst binders. Other active substances, for example platinum and/or palladium, can also be incorporated into alkylation catalysts to provide a metal hydrogenation function. Other catalysts capable of catalyzing alkylation can be produced by the deposition of agents covalently bound to, or entrained in, polymers on a solid surface not generally capable or poorly capable of catalyzing alkylation.
Various references that provide guidance in the composition, preparation/obtention and use of such catalysts are known. In this respect, reference in made to U.S. Pat. Nos. 5,491,277; 5,489,732; 5,345,028; and 5,304,698. The disclosures of these patents are incorporated by reference herein.
Solid alkylation catalysts affect alkylation kinetics. However, an alkylation catalyst does not effectively modify alkylation kinetics when at least one of a variety of conditions is satisfied. For example, an alkylation catalyst is not effective when, despite being in the presence of the alkylation reactants at the appropriate thermodynamic reaction conditions, the catalyst is deactivated. In another example, an alkylation catalyst does not effectively modify alkylation kinetics when the catalyst is under conditions such that not all of the alkylation reactants are available. Conditions in which not all of the alkylation reactants are present for the alkylation to take place will hereinafter be referred to as “the absence of alkylation.”
Reactants, intermediate reaction species, and alkylates of a variety of sizes and shapes can participate in a variety of alkylations. The shape and size selectivity of the zeolite is directly related to the shape and size of the channels in the zeolite. Accordingly, selection of the appropriate zeolite for any given alkylation will be determined by its structural characteristics. Structure, dimensions and pore characteristics of zeolites are provided in numerous sources, such as J. A. Martens, et al., Estimation of the void structure and pore dimensions of molecular sieve zeolites using the hydroconversion of n-decane, Zeolites 4, 98 (1984); W. Hölderich, et al., Industrial application of zeolite catalysts in petrochemical processes, Ger. Chem. Eng. 8, 337 (1985); W. Hölderich, et al., Zeolites: Catalysts for organic syntheses, Angew. Chem. Int. Ed. Engl. 27, 226 (1988); S. M. Csicsery, Catalysis by shape selective zeolites—Science and technology, Pure & Appl. Chem. 58(6), 841(1986); W. Meier, et al., Atlas of zeolite structure types (1988). For example, zeolite A, erionite, and chabazite are classified as small-pore zeolites; medium-pore zeolites include zeolites ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-48, NU-10, Theta 1, TS-1, and sihealite; and large-pore zeolites comprise faujasite, X-zeolite, Y-zeolite, mordenite, offretite, zeolite L, zeolite Omega, zeolites ZSM-4, ZSM-12, and zeolite Z.
Although the availability and properties of a variety of alkylation catalysts for heterogeneous catalysis have been known for some time, the deactivation of most of these catalysts remains a problem. A number of attempts to solve or ameliorate aspects of the catalyst deactivation problem have been made.
Catalyst Deactivation Problem
U.S. Pat. Nos. 4,605,811 (hereinafter the “'811 patent”) and 4,721,826 (hereinafter the “'826 patent”) disclose a process for restoring or maintaining the activity of heterogeneous catalysts for reactions at normal and low pressures. Catalyst activity is restored or maintained by using a pressure greater than the critical pressure of the fluid phase and a temperature higher than or equal to the critical temperature of the fluid phase. The process disclosed in these patents includes the reactivation of the catalyst at a pressure and temperature that are in the supercritical region of the reaction medium phase diagram. This requirement limits the range of substances that can be chosen to regenerate the catalyst, because the critical pressure and temperature must be within the pressure and temperature ranges for which the reaction conditions have been optimized, otherwise the reaction would proceed less efficiently or it would even not take place significantly. Furthermore, the substance that regenerates the catalyst must be compatible with the reactants and products because reactivation takes place while the chemical reaction proceeds. Reported times for catalyst reactivation according to the processes disclosed in the '811 and the '826 patents include 24 hours and 75 hours.
U.S. Pat. No. 5,304,698 (hereinafter the “'698 patent”) discloses a solid catalyzed supercritical isoparaffin-olefin alkylation process. The alkylation conversion conditions of this process include temperature and pressure that are, respectively, at least equal to the critical temperature and critical pressure of the component of highest concentration in the feed stock. These conditions are maintained over the entire course of the reaction until the catalyst is completely deactivated. The '698 patent does not disclose how to regenerate a completely deactivated catalyst. Furthermore, the isoparaffin containing feed is not to be contacted with the catalyst according to the process disclosed in the '698 patent under pressure and temperature conditions below the critical temperature and critical pressure of the isoparaffin component of highest concentration in the feed. The '698 patent teaches the use of conditions under which the component of highest concentration in the feed, being kept under supercritical conditions, prolongs the useful catalytic life of the crystalline microporous material through properties and behavior attributed to such component under supercritical conditions.
U.S. Pat. No. 5,310,713 discloses a process for regeneration of an alkylation catalyst with hydrogen. This process requires reactivation with hydrogen gas that can be mixed with liquid isobutane as a solvent. U.S. Pat. No. 5,491,277 (hereinafter the “'277 patent”) discloses a mixed-phase solid bed hydrocarbon alkylation process where “the exact manner of regeneration does not form . . . [part of] the process but is expected to include ‘washing’ of the catalyst with a liquid phase hydrocarbon such as isobutane or benzene, possibly at an elevated temperature and in the presence of some hydrogen to remove carbonaceous deposits.” Col. 6, 11. 28-33. The regeneration procedure disclosed in the '277 patent requires the presence of hydrogen with liquid isobutane that is supplied at a temperature of 100-150° C. as a solvent.
U.S. Pat. No. 5,489,732 discloses a fluidized solid bed motor fuel alkylation process in which the solid acid catalyst is continuously regenerated by removing it from the reactor and contacting it with hydrogen. In the first regeneration step, the hydrogen is dissolved in feed hydrocarbon and the catalyst is mildly regenerated. In the second regeneration step, the catalyst is separated from the liquid phase and regenerated with gaseous hydrogen at a temperature in the range 80-500° C. (preferably 100-250° C.). The regenerated catalyst is then fluidized with 38° C. isobutane and reintroduced to the bottom of the reactor. The average residence time of the regenerating catalyst in the liquid-phase hydrocarbon zone is 0.5-15 min, and the temperature and pressure in this zone are very near the reaction conditions for the alkylation.
The patents and other publications cited hereinabove are incorporated herein by reference in their entirety.
Catalyst Reactivation
The term “catalyst reactivation” will hereinafter be used to encompass catalyst regeneration and also catalyst reactivation. Catalyst reactivation refers to the treatment of a catalyst that renders it into a form in which it is suitable for its efficient use or re-use as a catalyst. “Reactivating agent” will hereinafter refer to a substance or mixture of substances that is used in catalyst reactivation.
The foregoing discussion indicates that it is highly desirable to provide heterogeneous catalysis that effectively replaces liquid acid catalysis in alkylation reactions. However, solid acid catalysts present problems associated with the catalyst's longevity and alkylate product quality.
Fouling substances that are generated in the alkylation process or that are introduced with the feed in the alkylation process fairly quickly reduce the number of the catalyst's active sites. Catalytic site reduction leads in turn to a reduction of the alkylation efficiency to a point such that the alkylation no longer takes place to any significant extent. Deactivated catalyst disposal would impose heavy burdens, such as those associated with waste disposal regulation compliance and the costs of resupplying the spent catalyst.
Methods employing supercritical fluids that are directed to the extension of the useful life of catalysts have not addressed the need to reactivate catalysts that have become deactivated. Furthermore, proposed methods for prolonging the longevity of alkylation catalysts rely on the maintenance of supercritical temperature and pressure conditions throughout the alkylation. This is a requirement that imposes a variety of limitations on the alkylation process, including a limited choice of reactivating agents and the possibly inefficient running of the alkylation.
According to one alkylation strategy, the temperature and pressure of alkylation conditions must be within narrow limits to procure the optimal thermodynamic and kinetic conditions and to avoid undesired byproducts and additional fouling agents. In those cases at least, the choices for the reactivating agent are typically very limited. Furthermore, only a very reduced number of substances that do not actually participate in the alkylation itself may have a critical pressure and a critical temperature that fall within the optimal pressure and temperature reaction conditions.
According to another strategy, the alkylation is run at a temperature and pressure high enough that they are within the supercritical conditions of at least one of the reactants. This reactant is then assigned the function of removing fouling agents and thus prolonging the longevity of the catalyst as an effective alkylation catalyst. However, the required critical pressure and critical temperature might be so high that they are detrimental to the alkylate quality. For example, such temperature and/or pressure conditions may favor undesirable side reactions, such as isomerizations, product cracking, olefin oligomerization, and coking, which might predominate over the desired alkylation. Product quality and high octane product yield are then significantly reduced. In addition, some of the undesired side reactions might contribute to the additional build up of fouling agents, thus aggravating the problem that was to be solved.
According to still another strategy, the alkylation catalyst is transferred out of the reactor for its total or partial reactivation. Reactivation is then accomplished by processes such as calcination, treatment with solvents, and elution with substances that dissolve and/or react with the fouling agents. The implementation of this strategy requires the substantial modification of reactor equipment or the complete removal and replacement of catalyst batches.
It would thus be desirable to provide a catalyst reactivation process that can rely on a substance that contains at least one of the alkylation reactants as reactivating agent, or some other substance that can be used as reactivating agent without detrimentally affecting the alkylation itself.
It would also be desirable to provide a catalyst reactivation process that can be carried out independently of the alkylation itself to reactivate a partially or totally deactivated catalyst under conditions such that the alkylation itself is not detrimentally affected. Furthermore, it would be desirable to provide a catalyst reactivation process that can effectively reactivate the catalyst regardless of the optimal pressure and temperature conditions at which the alkylation is run.
This reactivation process should rely on a reactivating agent that removes fouling agents by reacting with and dissolving them. In this way, the process' reactivating ability is considerably enhanced with respect to the reactivating ability of those processes that rely on the mere dissolution of certain fouling agents in the medium that extends the catalyst's useful life.